Note: Scale of y Axis on Both Figures Is Not the Same Frequency Tf = 750 µs 208 µs ≤ Tds ≤ 667 µs 62 µs ≤ Tus ≤ 521 µs TDD: G.Fast 105.93 MHz 20.8 µs DS US DS 2.22 MHz (a) Time 17.664 MHz DS3 FDD: VDSL 12 MHz US2 8.5 MHz FD D: V D SL DS2 5.2 MHz US1 3.75 MHz DS1 138 kHz (b) Time FIGURE 4. The duplexing methods for (a) G.fast (TDD system, 106-MHz profile) and (b) VDSL (FDD, 17-MHz profile) that separate upstream and downstream transmissions to mitigate NEXT in a DSL system. A typical TDD frame duration is 750 μs, comprising 36 symbols. The number of symbols for the downstream ranges from ten to 32 and for the upstream ranges from 25 to three, with a single guard symbol between them. US: upstream; DS: downstream. K Subcarriers 2K Samples ~ Y1 IFFT (User 1) ADC Y1 FFT (User 1) User 1 YK CPE 1 ADC DAC IFFT (User N ) Postcanceler (Tone 1) F1 Vectored Users (Tone 1) X1 Vectored Users (Tone K ) Y1 XK Channel CPE N X1 DMT Encoder (User N) XK K Subcarriers 2K Samples DAC X1 DMT Encoder (User 1) XK Postcanceler (Tone K ) FK FFT (User N) ~ YK ~ Y1 User N ~ YK YK PerTone FEQ PerTone FEQ Vectored Receive Processing at DP Single-User Transmission from Each CPE FIGURE 5. A schematic diagram of vectored receive processing in the upstream. There is no signal coordination among CPEs for joint transmit processing. At the DP, the received signals from CPEs at each tone are collected as a single vector, on which a canceler is applied. FEQ: frequency-domain equalizer. 146 IEEE SIGNAL PROCESSING MAGAZINE | September 2017 |